1. Field of the Invention
The present invention relates to an organic semiconductor material and an organic electronic device such as a field effect transistor. Particularly, it relates to an organic semiconductor material comprising a generalized porphyrin compound having a specific structure, and an organic electronic device employing such a material.
2. Discussion of the Background
Heretofore, as a field effect transistor (hereinafter sometimes referred to as FET) device, one employing, as a semiconductor layer, an inorganic semiconductor material such as a silicon (Si) or gallium arsenide single crystal, has been widely used. However, in the case of an inorganic material, it will be treated at a high temperature of at least 300° C. at the time of its production, whereby it is difficult to employ a plastic (or resin) for the substrate, and a large energy is required for its production. It requires a production process under vacuum, such as vapor deposition, sputtering or CVD, whereby it is difficult to produce a device having a large surface area. Further, it requires an expensive installation in its production line, thus leading to a problem of a high cost, etc.
Under the circumstances, an organic electronic device has been proposed wherein an organic semiconductor material is used for a semiconductor layer of an electronic device such as a field effect transistor, a light emitting diode or a nonlinear optical device. According to such a proposal, such a semiconductor layer can be prepared by a relatively low temperature process, whereby a plastic film can be used as the substrate, and there is a merit such that a device which is light in weight, excellent in flexibility and scarcely breakable, can be prepared. Further, it can be formed by a coating method or a printing method, whereby there is a merit such that a device having a large area can be produced at low cost without necessity of an expensive installation. Further, the organic material is rich in variation, and it is possible to basically change the properties of the material by changing its molecular structure, whereby there is a possibility that it is possible to obtain a device having a function which an inorganic material can not provide.
Organic semiconductor materials may be classified broadly into two types i.e. high molecular compound material (a polymer material) and low molecular compound material. With respect to each of them, there is a report i.e. on a device employing a conductive high molecular compound or a conjugated high molecular compound (JP-A-61-202467), or on a device employing a low molecular compound (JP2984370).
As such a high molecular compound material, a conductive polymer or a conjugated polymer is, for example, typical, and it has been attempted to use a conjugated polymer compound as it is, as a semiconductor, or to carry out switching by applying an electric field to introduce or withdraw ions (dopants) to or from a conjugated polymer compound. However, there have been problems inherent to a polymer, such that the solubility in a solvent is low, whereby a uniform coating fluid can hardly be obtained, the film is poor in uniformity or stability, defects attributable to incomplete structural portions are likely to result during the film formation, the purification is difficult, and the oxidation potential tends to be low, whereby the material is susceptible to oxidation. Thus, a material having high performance and high stability has not yet been found.
Whereas, in the case of the low molecular compound, the structure of the compound obtainable as a result of the synthesis is substantially predetermined, and various purification methods such as sublimation purification, recrystallization, column chromatography, etc. can be used. Thus, it is superior in that the purity is high, and a material having high performance and high stability can readily be obtainable.
As an example of such a low molecular compound material, an aromatic condensed hydrocarbon compound such as pentacene or an oligothiophene having 4 or more thiophene rings chained, which as formed into a film by vapor deposition, shows a mobility as high as amorphous silicon (a-Si), has been reported. However, such a low molecular compound tends to be oxidized although not so much as the high molecular weight compound, and there is a problem from the viewpoint of the stability. Namely, oxygen in the air is likely to be doped to the organic semiconductor film, whereby it is likely that the carrier density increases, and the leakage current increases or the mobility changes, whereby constant characteristics can hardly be obtainable.
Further, the low molecular compound can hardly be one whereby the characteristics of an organic compound are sufficiently utilized, since a coating process is hardly applicable thereto, and it is required to employ a film-forming method by vapor deposition which makes the production cost high. Further, if the low molecular weight compound is formed into a film by coating of its solution, a uniform film can hardly be obtainable, since it will have a granular structure by crystallization, and thus, there will be many cases wherein there is a problem in the film forming properties.
For example, applications of phthalocyanines to field effect transistors have been reported (JP-A-11-251601, JP-A-2000-174277, Appl. Phys. Lett., vol. 69 (1996), p. 3,086). However, phthalocyanines are usually insoluble in solvents, to prepare such devices, and it is necessary to carry out film formation by a vacuum vapor deposition method.
Under the circumstances, methods have been reported in recent years wherein a precursor for a low molecular compound having a high solubility in a solvent, is dissolved in a solvent or the like, then formed into a film by a coating process and then converted to a semiconductor to obtain an organic semiconductor film, so that a field effective transistor is thereby prepared. For example, there are cases wherein pentacene or analogous aromatic hydrocarbons are employed (Science, vol. 270 (1995) p. 972, Optical Materials vol. 12 (1999), p. 189, J. Appln. Phys. Vol. 79 (1996) p. 2,136).
Here, the operation characteristics of a field effect transistor are determined mainly by the carrier mobility μ or electroconductivity of the semiconductor layer, the capacitance Ci of the insulating layer, and the construction of the device (such as the source•drain electrode distance L and width W, the thickness d of the insulating layer, etc.). Among them, it is important that the carrier mobility μ (hereinafter sometimes referred to simply as the mobility) of the semiconductor material to be used for the semiconductor layer, is high. With respect to pentacene, a case where the mobility is 0.2 cm2/Vs depending upon the condition of the film, has been reported. However, the mobility demonstrated by an actual application to a device has been at a level of 10−2 cm2/Vs, and the mobility in the practical use is not yet high. Further, from the pentacene precursor in this case, a tetrachlorobenzene molecule will be detached, but tetrachlorobenzene is not only hardly removable from the reaction system as the boiling point is high, but also problematic in view of its toxicity.
Meanwhile, as a material for an optical device to obtain a photoelectric current or photoelectromotive force, a porphyrin compound has been studied, and an application of benzoporphyrin to a solar cell is disclosed in JP-A-9-18039. However, its carrier mobility is low, and when the mobility is calculated from the carrier density and the resistivity disclosed in Examples, it is still at a level of 1.3×10−6 cm2/Vs even at the maximum. Since the mobility is so low, the study on the application of the porphyrin compound has been limited to an optical device, and no application to an organic electronic device has been observed wherein all mobility or electromobility is positively utilized.
As described above, an organic semiconductor material has various characteristics which are not observed with an inorganic semiconductor material. However, organic semiconductor materials having relatively high performance, such as phthalocyanines, pentacenes or oligothiophenes, are all restricted in that the process for their production has been limited to a vapor deposition process which is highly costly. Therefore, it is desired to obtain an organic electronic device which can be produced by a simpler process and which, at the same time, has practical characteristics.
Accordingly, an organic semiconductor material which has high carrier mobility and stability and which can be formed into a film by a simple production process such as a coating process, and an organic electronic device employing such an organic semiconductor material, have been desired.